In general, an image sensor is a semiconductor device that converts an optical image into an electrical signal. The image sensor is typically classified as a charge coupled device (CCD) image sensor or a complementary metal oxide semiconductor (CMOS) image sensor (CIS).
A unit pixel of such a CIS includes a photodiode and a metal oxide semiconductor (MOS) transistor. In operation, the CIS sequentially detects an electrical signal of the unit pixel in a switching manner to generate an image.
To improve optical sensitivity of the CIS, microlenses are used to condense light towards the photodiodes. Color filters are also included in certain CIS to produce a color image.
The color filters and the microlenses are formed on a pixel array substrate including pixels.
The microlenses are formed in a dome shape by sequentially performing an exposure process, a development process, and a reflow process to a photosensitive organic material.
In the case where the microlenses are formed through a single patterning process followed by a reflow process, adjacent microlenses may have gaps therebetween. Here, light passing through the gaps may be detected by a non-corresponding photodiode disposed on the pixel array substrate, which produces noise and crosstalk.
The gaps form between microlenses when wide areas between the microlenses in the patterning process for the microlenses are not sufficiently reduced in the reflow process.
As a matter of course, the gaps can be reduced in the reflow process. However, an excessive reflow process causes adjacent microlenses to merge with each other, which results in a bridge and merging of microlenses.